Known from the state of the art, for example, from WO 03/023536 A1, WO 2009/060000 A1, WO 2009/063053 A1 and EP 0940738 A2, is to use radio units in plants of process automation technology for easier servicing of field devices. There are solutions, in the case of which the field devices have integrated radio units. It is, however, also known to retrofit field devices with radio units, so that the field devices can then be serviced wirelessly.
Energy supply of the field devices as well as also the radio units occurs, in such case, for example, via an electrical current loop, to which one or more field devices and/or one or more radio units are connected. These electrical current loops involve, in such case, as regards hardware, two- or multiconductor arrangements. The supply of the electrical current loop with electrical energy occurs, for example, via a feeding device, such as, for example, a power supply, which delivers the electrical voltage required for establishing an electrical current value in the electrical current loop. If the electrical energy present in the electrical current loop is not sufficient to supply all field devices connected to the electrical current loop, it is also possible to provide separate voltage supplies via additional feeding devices for the individual field devices, respectively the individual radio units. To this end, it is, for example, known from the state of the art, for example, as represented by DE 102004020393 A1, to integrate a battery into the radio unit, so that the radio unit does not need to be supplied with electrical energy via the electrical current loop.
In industrial plants, consequently, due to the large number of installed field devices, often a number of feeding devices are required, in order, on the one hand, to assure communication via the electrical current loop and, on the other hand, to assure operational capability of the field devices connected thereto.
Known from the state of the art are, moreover, fieldbus systems, such as, for example, the HART® protocol in so-called multidrop operation, in which a number of field devices are connected, for example, with one another, via the aforementioned electrical current loop. Additionally, such fieldbus systems can make use of a radio unit, which is integrated in the bus system, in order to service the field devices connected to the bus system or to transmit their measurement data. Such installations are, in given cases, arranged remotely from the operating personnel, so that servicing and/or measured value transmission occurs preferably via the radio unit. However, exactly a long distance transmission of radio signals, for example, via GSM or similar standard known for mobile radio communication, is energy intensive, so that such radio units require a special energy supply.